There are many situations where a dimension measurement is desired at locations that are difficult to access because of interfering structure or hazardous due to hostile environment. One situation that poses both problems is the measurement of gap dimensions within nuclear reactor fuel bundles. That is, it is important to measure the gaps between fuel rods of fuel bundles for both pre-irradiation and post-irradiation conditions. In the latter case, the rod-to-rod gap dimensions must be obtained under water in the presence of extremely high radiation fields.
Presently, rod gap measurements are made using a device known in the industry as a "SULO" probe available from Intricate Machinery of Attleboro, Mass. This probe includes a pair of thin stainless-steel leaf springs with strain gauges sandwiched therebetween. The electrical signals produced by the strain gauges are proportional to the flexure of the leaf springs as they conform to the rod gap and thus can be processed to provide a remote indication of the rod gap dimension.
The SULO probe, while capable of providing rod gap readings of acceptable accuracy, is relatively expensive, requires frequent recalibration, and has a short service life.